Embodiments of the invention relate generally to diagnostic imaging and, more particularly, to a system and method of acquiring multi-energy data for material decomposition.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped or cone-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis, which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include an anti-scatter grid or collimator for rejecting scattered x-rays at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom. Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
In a given energy region relevant to medical CT, two physical processes dominate the x-ray attenuation: (1) Compton scatter and the (2) photoelectric effect. These two processes are sensitive to the photon energy and hence each of the atomic elements has a unique energy sensitive attenuation signature. Therefore, the detected signals from two energy regions provide sufficient information to resolve the energy dependence of the attenuation of the material being imaged. Furthermore, detected signals from the two energy regions provide sufficient information to determine material attenuation coefficients in terms of Compton scatter and photoelectric effect. Alternatively, the material attenuation may be expressed as the relative composition of two hypothetical materials. As understood in the art, using a mathematical change of basis, energy sensitive attenuation can be expressed in terms of two base materials, densities, effective Z number, or as two monochromatic representations having different keV. In some cases, such as in the presence of materials with K-edges in their attenuation profile, more than two basis functions may be preferred. And, as known in the art, one known method for material decomposition image reconstruction reconstructs a material basis image based on the two base materials.
A CT imaging system may therefore include an energy sensitive (ES), multi-energy (ME), and/or dual-energy (DE) CT imaging system that may be referred to as an ESCT, MECT, and/or DECT imaging system, in order to acquire data for material decomposition or effective Z or monochromatic image estimation. ESCT/MECT/DECT provides energy discrimination. For example, in the absence of object scatter, the system derives the material attenuation at any energy based on the signal from two relative regions of photon energy from the spectrum: the low-energy and the high-energy portions of the incident x-ray spectrum.
Such systems may use a direct conversion detector material in lieu of a scintillator. One of the ESCT, MECT, and/or DECT imaging systems in an example is configured to be responsive to different x-ray spectra. Energy sensitive detectors may be used such that each x-ray photon reaching the detector is recorded with its photon energy. One technique to acquire projection data for material decomposition includes using energy sensitive detectors, such as a CZT or other direct conversion material having electronically pixelated structures or anodes attached thereto. However, such systems typically include additional cost and complexity of operation in order separate and distinguish energy content of each received x-ray photon.
In an alternative, a conventional scintillator-based third-generation CT system may be used to provide energy sensitive measurements. Such systems may acquire projections sequentially at different peak kilovoltage (kVp) operating levels of the x-ray tube, which changes the peak and spectrum of energy of the incident photons comprising the emitted x-ray beams. A principle objective of scanning with two distinctive energy spectra is to obtain diagnostic CT images that enhance information (contrast separation, material specificity, etc.) within the image by utilizing two scans at different polychromatic energy states.
One technique has been proposed to achieve energy sensitive scanning including acquiring two scans at, for instance, 80 kVp and 140 kVp. The two scans may be obtained (1) back-to-back sequentially in time where the scans require two rotations of the gantry around the subject that may be hundreds of milliseconds to seconds apart, (2) interleaved as a function of the rotation angle requiring one rotation around the subject at each kVp, or (3) using a two tube/two detector system with the tubes/detectors mounted ˜90 degrees apart, as examples.
However, such systems may be prone to image quality issues and/or have costs associated therewith. For instance, in the first example (1), when two scans, at low and high energy, are obtained back-to-back sequentially in time, the “hundreds of milliseconds to seconds” separation in time between the two scans can cause image artifacts and blurring due to patient motion and registration errors. Also, the scan typically takes 2× the amount of time compared to a conventional monochromatic imaging session, and the amount of radiation dose can be significantly increased because the two sets of data are acquired at each relative axial location. In the second example (2), when interleaving, system capacitance and other factors can cause a reduced amount of energy separation to occur during the rapid alternation between low and high energy states. That is, although example (2) may be desirable for a number of reasons (rapid data acquisition, low dose, shortened scanning sessions, and low cost, as examples), desired energy separation between low and high may not be obtained because the system may not reach its desired energy state in the short period of time before the system is triggered to switch to the next energy state. In the third example (3), a two tube/two detector system tend to be prohibitively expensive. And registration of high and low energy scans also can result in misregistration due to the time lapse between the first and second tube scanning, leading to image artifacts and blurring.
Thus, there is a need to acquire dual or multi-energy data having little or no data misregistration between high and low energy shots, and in a cost-competitive fashion. It would therefore be desirable to design a system and method for acquiring multi-energy data for material decomposition.